1. Field of the Invention
The field of the invention generally relates to thermoplastic polyurethanes and thermoplastic polyureas having properties similar to those of castable or cross-linked polyurethanes or polyureas. The field of the invention also includes methods of making the same.
2. Description of the Related Art
There currently are a number of commercialized products made from polyurethanes and polyureas. Typically, these products made from either thermoplastic polyurethanes (or polyureas) or thermoset polyurethanes (or polyureas). Thermoplastic polyurethanes generally have linear molecular structures and are able to flow freely at elevated temperatures. For this reason, thermoplastic polyurethanes are preferred for products which are produced by injection molding or other extrusion techniques, where flowability of the reactants are of paramount importance. Unfortunately, thermoplastic polyurethanes typically exhibit poor performance characteristics with respect to abrasion, tensile strength, rebound, and compression set compared to castable polyurethanes.
In contrast to current thermoplastic polyurethanes, thermoset polyurethanes have particularly good characteristics with respect to abrasion, tensile strength, rebound, and compression set. Thermoset polyurethanes generally have a network structure that incorporates irreversible chemical cross-linking. The downside of thermoset polyurethanes is that the irreversible chemical cross-linking reaction makes it unsuitable for use in injection molding and extrusion applications. Typically, thermoset polyurethanes are formed using a casting process. Unfortunately, casting processes require costly equipment and usually involve a large number of processing steps. Casting is thus a less efficient and more expensive method of producing polyurethane-based and polyurea-based products as compared to injection molding and extrusion systems.
In a typical process for making a thermoset (i.e., castable) polyurethane, a di-isocyanate component is first pre-polymerized with a polyol having either a polyester or polyether backbone. The remaining di-isocyanate of the pre-polymer is reacted with a chain extender or a cross-linking agent or a blend of cross-linking agents. Catalysts are added to control the reaction rate. If the cross-linking agent has a dihydroxy functional component, a polyurethane will be formed. If the cross-linking agent has diamine functionality, a polyurea is formed.
With respect to thermoplastic polyurethanes, a diol or polyol is reacted with an isocyanate. This reaction typically takes place in large commercial reactors. As stated above, thermoplastic polyurethanes, while not cross-linked, are usable in injection molding and other extrusion methods. Because of the lack of cross-linking, these materials have abrasion, tensile, and compression set properties that are not as good as thermoset polyurethane or polyurea systems.
There thus is a need for a thermoplastic polyurethane or polyurea material which exhibits good abrasion, tensile strength, rebound, and compression set characteristics which are similar to those found in thermoset urethanes. Such a material could be produced using conventional injection molding and/or extrusion techniques, thereby reducing the cost of manufacture for the material.
The conventional solution has been to add a cross-linking agent to a thermoplastic resin, either post-injection molding or pre-injection molding. Adding the cross-linking agent pre-injection molding allows for a greater dispersion of the cross-linking agent within the thermoplastic resin material. However, many cross-linking agents have low boiling temperatures and will “flash off” prior to dispersion, especially at room temperatures. Trying to add other agents to resolve the flash off problems leads to other processing issues, especially with injection molding machines which melt processing materials (thermoplastic resins) and inject the melted materials under high pressures to form various articles, which can lead to a build up of materials in the injection molding barrel.